The excision of diseased or abnormal tissue from the body traditionally has been termed an “invasive” one. In carrying out invasive surgery, medical practitioners generally have resorted to the use of sharpened edge tools and, for about six decades, additionally, forms of electrosurgery. In the latter regard, a somewhat pioneer electrosurgical device was developed by William T. Bovie. This early device, described, for example, in U.S. Pat. No. 1,813,902 issued on Jul. 14, 1931 entitled “Electrosurgical Apparatus” and its successors have met with acceptance over the years within the surgical community to the extent that current versions are referred to as the “Bovie”.
For both traditional excision approaches, injury generally occurs to surrounding or peripheral and healthy tissue. While certain of such injuries are apparent, others have been reported which are more subtle. Conventional removal of malignant tumor, as well as more simple biopsy procedures have been reported to generate “seeding” or spreading or metastasizing cancer in the body. In addition to patient discomfort and longer recovery periods, more invasive surgical procedures are reported to be accompanied by a period of immunosuppression, a condition increasing the risk of disease spread. See the following publications in this regard:                “Impaired Production of Interlukin-2 after Surgery,” T. Akiyoshi, et al., Clin. Exp. Immunology, Vol. 59, pp 45-49, 1985.        “The Influence of Surgical Operations on Components of the Human Immune System,” T. Lennard, et al., British J. of Surgery, Vol. 72, pp 771-776, 1985.        
Less invasive alternatives to conventional surgical procedures have been and continue to be investigated, particularly as the clinical detection of tumor or tissue abnormalities has become more refined. For example, current imaging systems (mammography, ultrasonographs, MRI) may detect and locate very small tumor or tissue abnormalities sized at the level of a millimeter. Where such tumor is detected, for example, in the breast, biopsy procedures employing fine needle aspiration techniques may be utilized. Retrospective investigation, however, has determined that about 80% of such biopsied tissue is benign. Where malignancy is determined, the biopsy procedure risks the above-noted seeding or metastasization opportunities. Excision of even the smaller aberrant tissue zones typically is both traumatic to the patient and relatively cost intensive. The latter cost aspect also is present with conventional needle biopsy procedures.
Particularly where small tumors or tissue abnormalities are encountered, investigators have looked to potentially less invasive and thus less costly and less traumatic procedures. For example, if a smaller tumor can be biologically destroyed in situ so as to become ischemic or necrotic, the resultant small zone of dead tissue eventually will be physiologically eliminated by resorption.
One approach to carrying out an in situ destruction of such smaller targeted tissue zones has been to thermally affect the volume of aberrant tissue. Such an approach may involve either cooling or heating the target tissue to the point of irreversible cell death or necrosis. For the former, cooling approach, reference is made to following publication:                “Requisites for Successful Cryogenic Surgery of Cancer,” H. Neel, et al., Arch. Surg., Vol. 102, pp 45-48, 1971.        
The latter approach, that of inducing therapeutic hyperthermia generally is a less invasive one. A rather broad variety of technical modalities have evolved to elevate the temperature of tissue. For example, biological tissue volumes may be heated by inductive, radiant, contact or joulean based techniques. While these hyperthermic approaches exhibit potential advantage over the highly invasive surgical modalities, limitations to their use have been identified. Inductively based systems, certain of which are described in U.S. Pat. Nos. 5,251,645 and 4,679,561 perform by passing high frequency electromagnetic radiation through tissue. This is achieved by passing the medication between two external electrodes positioned adjacent the patient's skin. A drawback of such an approach to therapeutic hyperthermia resides in the heating of a relatively large volume of tissue at elevated temperatures for extended intervals of time. Typically with this practice, tissue is heated to temperatures of 6° C. to 10° C. above normal body temperature for periods of twenty minutes or more to achieve necrosis. The systems generally do not allow the volume of tissue to be well defined, i.e., the treatment is inaccurate, resulting in either insufficient necrosis or excessive necrosis extending into surrounding healthy tissue. As a consequence, practitioners have looked to combining prolonged heating of tissue with chemotherapy or radiation therapy modalities.
Interstitial thermotherapy has become an important alternative to invasive surgical methods. In general, six thermotherapy modalities have been developed for heating or cooling tissue. They are identified as: (1) radiofrequency heating, (2) microwave heating, (3) laser heating, (4) ultrasound heating and (5) cryogenic cooling. Radiofrequency heating procedures are categorized as direct and indirect. The latter, indirect, approach involves the placement of metal wires or pellets (which may be autoregulated) in the target tissue and then externally applying an R.F. field.
The above six modalities involve either of two methods of temperature alteration in tissue, to wit, conduction and diffuse or distributed heating of targeted tissue. Conduction may be of heat from or to a device or instrument and is characterized as a slow process since thermal diffusion through tissue is a somewhat slow phenomenon. This can lead not only to longer treatment periods but uncertainty in the size and shape of the final lesion, again a problem of treatment accuracy. Such conduction-limited modalities include: indirect radiofrequency heating, laser heating, and cryogenic cooling. Conduction-limited therapeutic heating of tissue using radiant sources is described, for example, in U.S. Pat. Nos. 5,284,144; 4,872,458; and 4,737,628. Radiant sources, such as lasers, produce localized heating of tissue, but do not permit the affected volume to be predetermined, a priori. Other conduction-limited contact heating approaches have been used for inducing therapeutic hyperthermia as are described in U.S. Pat. Nos. 4,979,518; 4,860,744; 4,658,836; and 4,520;249.
Diffuse or distributed heating of targeted tissue is distinctly different from the above-described conduction-limited method. This approach has the potential advantage that the target tissue can be heated to a desired cauterization temperature within relatively shorter interval of time. Cauterization procedures involve bringing targeted tissue to a temperature within a predetermined temperature range for a duration resulting in irreversible cell death. However, while representing a procedure exhibiting much promise, investigators have encountered obstacles in its implementation. In this regard, the volume of tissue cauterized is generally more difficult to control for systems incorporating microwave or ultrasound procedures, inasmuch as these procedures depend upon the radiation of tissue-heating energy into a volume of tissue from an emitting transducer or antennae system. The precise size of any resulting lesion depends upon the duration of treatment as well as the microwave or ultrasound responsiveness of the targeted tissue. In this regard, investigators have looked to the placement of one or more temperature sensors within the treatment field or have looked to the measurement of electrical impedance to assess the extent of the volume of cauterized tissue to determine an end point termination of the therapy. The problem of treatment accuracy again is posed. See generally, U.S. Pat. Nos. 5,122,137; 4,776,334; and 4,016,866. A direct measurement of tissue impedance is described, for example, in U.S. Pat. Nos. 5,069,223 and 4,140,109. These procedures are complex and somewhat costly. Of the diffuse or distributed heating approaches, electrosurgical techniques hold promise for both precise and predictable cauterization of targeted tissue volume, as well as a rapidity of the treatment process. Devices and technology representing this category are described, for example, in U.S. Pat. Nos. 5,728,143; 5,683,384; 5,672,173; 5,672,174; 5,599,346; 5,599,345; 5,486,161; 5,472,441; 5,458,597; 5,536,267; 5,507,743; 4,486,196; 4,121,592; and 4,016,886. See also, PCT Application WO 96/29946.
Electrosurgical instruments generally perform in either of two operational modes, monopolar or bipolar. In the monopolar mode, electric current is conducted between a relatively small active electrode and a large return electrode located a distance from the active electrode. Because in the monopolar mode, current density in tissue decreases as the square of the distance from the active electrode, it is more difficult to treat more than very minimal volumes of targeted tissue as well as to maintain the volumetric accuracy of such treatment. Notwithstanding such a surface related operational limitation, the monopolar devices are quite efficient as electrosurgical cutting tools and for the purpose of carrying out a coagulation at the surface of tissue being cut. Each approach involves a different waveform but both are surface related and involve arcing between the instrument tip and the tissue being affected.
The bipolar mode of electrosurgical (joulean) heating involves passing current between tissue disposed between two electrodes of similar surface area. To effect cauterization of targeted tissue, this electrosurgical heating technique has been implemented with instruments which deploy pointed, flexible fine wire or needle-like electrode-functioning stylets directly into the targeted tissue. This calls for a mechanical system carrying out tissue penetration with these fine deployed stylets which necessarily will have a small surface area per unit length of the electrode. As a consequence, the permissible current flux flowing between the electrodes is significantly limited inasmuch as excessive current densities will cause desiccation of tissue immediately adjacent the electrodes which defeats the procedure. This follows, inasmuch as the desiccated tissue adjacent the electrode will then exhibit a very high electrical impedance which prevents further tissue heating and thus limits the volume of tissue which can be treated to the point of effective cauterization. For this reason, the fine needle or stylet techniques heretofore employed have been observed to require a treatment duration of ten to fifteen minutes for larger lesions. Further, a temperature monitoring of the fine electrode and even the infusion of conductive fluids is called for to reduce impedance between the electrodes and surrounding tissue. Additionally, practice with the needle extruding mechanisms have shown them to be difficult to deploy, the practitioner having less than desirable information as to the exact positioning of the fine electrode stylets. For example, these wires will deflect in the procedure of insertion into the targeted tissue in dependence upon their degree of flexibility as well as upon the varying density characteristics of abnormal tissue sought to be cauterized. Placement identification or observation procedures using conventional imagining systems is hindered because of the highly diminutive surface area of the electrodes themselves. In this regard, such imagining systems fail to “see” the electrodes. As a consequence, the targeted tissue is either under-treated or the treatment procedure extends cauterization excessively into adjacent healthy tissue, i.e., it encroaches excessively beyond the targeted tissue volume. Treatment accuracy again remains problematic. Bipolar mode electrosurgical procedures are described, for example, in U.S. Pat. Nos. 5,720,744; 5,403,311; 5,122,137; 4,920,978; 4,919,138; and 4,821,725, while fine needle electrode technologies are set forth, for example, in U.S. Pat. Nos. 5,470,309; 5,370,675; 5,421,819; 5,470,308; and 5,607,389.
Investigators also have looked to the destruction or control of tumor by the devitalization or vascular interruption of oxygen and nutrient ingress to targeted tissue volumes. Resultant cell death or necrosis again may be accompanied by its physiologically natural absorption by the body. As before, while this general approach to tumor management holds promise, the practical aspects of control over the targeted tissue volume using minimally invasive tactics has remained elusive. See generally; Denekamp et al., “Vascular Occlusion and Tumor Cell Death,” Eur. J. Cancer and Clinical Oncology, Vol. 19 No. 2, pp 271-275 (1983).
As is essentially the case in all remotely guided procedures, the process for carrying out an incision, for example, of a volume of targeted tissue is difficult. This difficulty is particularly in evidence where an incision is called for which does not invade the targeted tissue volume, extending only about its periphery.
Highly controlled and accurate RF electrosurgical cutting promises to enjoy a number of surgical applications beyond topics such as vascular isolation of tissue volume. For example, rather basic intravascular catheter guided monopolar electrodes have been employed as therapy for a variety of cardiac dysrhythmias. The therapy involves maneuvering of a monopolar electrode to sites of arrhythmogenic myocardium to carry out an ablation of heart muscle at discrete areas. While the therapy has demonstrated high therapeutic effectiveness, the treatment procedure is somewhat primitive, substantial volumes of tissue at the inner wall of the heart being destroyed until the aberrant conduction pathway is eliminated or blocked by the resultantly necrosed tissue. R. F. catheter ablation techniques also have been used to treat ventricular tachycardias, atrial flutter, ectopic atrial tachycardia, and sinus node reentry, albeit with lower success rates. These techniques are still evolving and, as is apparent, a technique for accurately forming a controlled linear lesion of known and minimal dimension will represent a beneficial advance in the therapy. See generally: Wood et al, Radiofrequency Catheter Ablation for the Management of Cardiac Tachyarrhythmias, Am J Med Sci 1993; 306(4):241-247.